If President Clinton's impeachment trial were handled like any other civil or criminal proceeding, most of the 100 senators in the jury would have already been struck for cause: They all know the defendant, some of them quite well, and many Democrats have worked with him, and Republicans against him.

But party loyalty alone cannot explain how the Senate operates. And as the trial resumes Thursday, the White House and the House Republican prosecutors will be looking to peel supporters away from the other side, pitching their arguments at the senators who – for political or personal reasons – might break with prevailing party orthodoxy. The White House will be eyeing the 19 Republicans who are up for reelection in 2000, several of whom – such as Pennsylvania's Rick Santorum or Washington's Slade Gorton – are from states that Clinton won in 1996. Clinton advisers will also be watching a handful of GOP moderates who have supported Democratic initiatives in the past. Can they be induced to break ranks once again?

Meanwhile, the prosecution will aim its arguments at those moderate Democrats from the South – Florida's Bob Graham is a prime example – where Clinton's popularity slumps. Prosecutors will also focus on a handful of mavericks who have broken ranks with other Democrats on issues related to impeachment, and might do so again.

Above all, everyone will have to be careful not to offend the "institutionalists" of both parties who will not tolerate behavior that they believe insults the Senate or evidence that turns the trial into a spectacle.

"It is important that we conduct ourselves here in a way that will reflect well on the Senate and reflect well on the Constitution, and that will preserve for our future generations the very important concept" of impeachment, said Sen. Robert C. Byrd (D-W.Va.), the Senate's premier institutionalist. "We must not tarnish that provision."

Far from the typical cast of nameless, faceless jurors in a high-profile case, Clinton's panel is an elite powerhouse of politicians, many of whom are dominant figures in their respective parties.

The Senate is overwhelmingly white, male and middle-aged to elderly. Fifty-five senators are lawyers and a quarter are businessmen and former bankers. Dozens are millionaires. The average age is 58.3; there are eight women; one Japanese American; one native Hawaiian; one American Indian; no blacks and no Hispanics. It is divided between 55 Republicans and 45 Democrats.

Until 1913, senators were appointed by state legislatures, with the idea, Byrd said, that they would be "insulated" from the rough-and-tumble of direct election politics, and "wouldn't be swayed by the frenzy."

The 17th Amendment to the Constitution, providing for direct election of senators, was "a sea change," Byrd continued. "Today we have senators who have their ears very much to the ground. They are not as insulated or independent."

Still, under Senate rules, anyone can make a motion, propose an amendment or give a speech at any time, so no one can run roughshod over anyone else, and all senators have to go along to get along to some degree. "It's like the Old West, when everyone carried a six-shooter," Sen. Phil Gramm (R-Tex.) said. "But they called each other 'Mister.'"

Thirty years ago the Senate was a relatively congenial place where conservative southern Democrats and liberal "Rockefeller Republicans" provided a comfortable middle ground where senators could make common cause.

Both these species have dwindled today, replaced by a new generation weaned on attack ads and negative campaigning and well-blooded in the ideological wars that have dominated the political landscape in recent years.

Deals on the budget, the environment and foreign policy are still possible, in part because of long-standing, across-the-aisle alliances and friendship, but also because of the political imperative that the two parties make an occasional show of bipartisanship.

But in the case of impeachment, where the political stakes for individuals and the two parties are so high, those familiar guideposts are less useful in assessing how the jurors are likely to react.

"Nothing is a given here," added Thomas Mann, a scholar with the Brookings Institution. "They don't move in blocs on this issue, except for the core conservatives who seem to be trying to string this thing out in hopes the next data dump will finally bring the public to its senses and bring an end to the Clinton presidency."

Most Republicans appear to favor calling witnesses as part of the trial; most Democrats do not. Most Democrats favor censure as an alternative to the removal of the president from office; most Republicans say they oppose censure, or are at best uncomfortable with it.

Still, even the most cursory profile of the Senate quickly demonstrates that the choices are almost never "either-or."

While there are Republicans such as Conrad Burns (Mont.), James M. Inhofe (Okla.) and Jesse A. Helms (N.C.) who want to stay true to the conservative cause, there are other conservatives such as John Ashcroft (Mo.), Santorum and Gorton, whose zeal is conditioned by the knowledge that they are among the Senate's most vulnerable: incumbents running in 2000 from states that Clinton carried in his 1996 reelection.

And while some Democrats such as Tom Harkin (Iowa) and Charles E. Schumer (N.Y.) have left little doubt of their disdain for the prosecution's case – Harkin called it "a pile of dung" – others, such as Bob Kerrey (Neb.) and Joseph I. Lieberman (Conn.), have never made a secret either of their lukewarm feelings for Clinton or their disgust with his behavior.

Some Republicans, knowing they are embarked on the impeachment trial of a president whose approval ratings top 70 percent, must contend with the real danger that a protracted, ugly trial could cost them the White House and their majorities in the House and Senate in 2000.

Chief election strategist Mitch McConnell (Ky.) said he thinks "it's going to have zero impact" because it will be soon forgotten. But can moderate Republicans from strong Democratic states such as Susan Collins (Maine), John H. Chafee (R.I.) and James J. Jeffords (Vt.) survive a hard-line strategy?

For their part, some Democrats have to worry that White House arrogance or new, damning evidence could send Clinton's approval ratings into the tank and take the party's 2000 chances along with them. Graham has said that he is not categorically opposed to witnesses. Can Minority Leader Thomas A. Daschle (D-S.D.) and his election strategist, Robert G. Torricelli (N.J.), afford unalloyed loyalty toward a badly damaged president?

Senators today worry about reelection, and, in the television age, worry about how they look and about everything they say. They can fly home easily, so they don't need to hobnob with each other on weekends. They are more into themselves, and know less about each other. Familiarity used to breed comity. Now there is less of it.

Warren B. Rudman (R-N.H.), who retired in 1993, noted that the Senate today contains "huge numbers of former members of the House," many of them Republicans who "had a very unpleasant experience" when the House was dominated by the Democrats.

The new Republican senators came in with an "attitude," and the new Democrats, fresh from a few years of abuse at the hands of the House's relatively new Republican majority, are just as tough.

Both Majority Leader Trent Lott (R-Miss.) and Daschle are former House members, but the fireworks come from newer arrivals such as Republicans Santorum, Inhofe and Jon Kyl (Ariz.), and Democrats Torricelli, Barbara Boxer (Calif.) and Richard J. Durbin (Ill.).

Rudman said Senate collegiality has also been damaged by the advent negative political campaigns. "It is very difficult to be a collegial colleague with someone who got there with intensely negative or maybe false campaigning against someone who was a friend of yours," Rudman said.

But you have to try. "The institution [Senate] makes each of us dependent on others to survive," Durbin said. "I've worked with senators [with whom] I'm poles apart in philosophy, and we've become social friends."

Across-the-aisle friendships have always played a crucial role in greasing the machinery of the Senate. The customary greetings among senators – the gentle pats on the back and tugs at the elbow – more often than not are gestures of genuine affection. "It's an intensely personal place," said Sheila Burke, chief of staff to then-Senate Majority Leader Robert J. Dole (R-Kan.).

Liberal Edward M. Kennedy (D-Mass.) and conservative Orrin G. Hatch (R-Utah), two longtime friends, repeatedly teamed up to pass important health care, child care and AIDS treatment legislation. John McCain (R-Ariz.) and Russell Feingold (D-Wis.), two other friends, championed campaign finance reform together in the face of overwhelming resistance in the Senate.

Burke, now the executive dean of the Kennedy School of Government at Harvard, said that for all its quirks and eccentricities, the Senate is a remarkably resilient and strong institution that can "weather the extremes on both sides."

"There have been a number of events that deeply divided them," she said, from the controversy over the "Keating Five" senators who were implicated in a savings and loan scandal to the Clarence Thomas-Anita Hill hearings. For better or worse, the Senate "got through them and moved on," Burke said, just as it will with the impeachment trial.